(core) User language switcher

Summary:
New language selector on the Account page for logged-in users.
New icon for switching language for an anonymous user.

For anonymous users, language is stored in a cookie grist_user_locale.
Language is stored in user settings for authenticated users and takes
precedence over what is stored in the cookie.

Test Plan: New tests

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3766

app/common/gristUrls.ts

@@ -585,6 +585,9 @@ export interface GristLoadConfig {
 
   // Email address of the support user.
   supportEmail?: string;
+
+  // Current user locale, read from the user options;
+  userLocale?: string;
 }
 
 export const HideableUiElements = StringUnion("helpCenter", "billing", "templates", "multiSite", "multiAccounts");